Modulating system



Aug. 8, 1944. H. M. CROSBY 2,355,422

- MODULATING SYSTEM Filed Nov. 2, 1942 I Inventor: Howard M. Cr'osbg,

Patented Aug. 8, 1944 PATENT orr cs mo um'rm'o sYsrnM Howard M. Crosby, Schenectady, N.'Y., assignor to General Electric New York Company, a corporation of Application mumm- 2, 1942, Serial No. 464,163

Claims. (0!. 119-1115) This invention relate to modulating systems and more particularly tomodulating circuits for radio telephone transmitters.

In certain cases of radio telephonic transmission it may be desirable to vary the radio'frequency power output of the transmitter. For example, a radio broadcasting station may operate on a given power output during the daylight hours and on a reduced power output during th evening.

In a conventional anode-modulated amplifier circuit the radio frequency output of the oscillator or radio frequency amplifier electron discharge device varies with the second power of any voltage applied to its anode circuit and the anode circuit impedance of the discharge device is constant. By inserting a resistance in series with the anode circuit, a reduction in D. C, anode voltage is obtained which reduces the radio frequency output according to the amount of voltage drop through the resistance. At the same time the applied audio or modulating voltage is reduced in. the same proportion so that the original percentage of modulation of the radio frequency output is efiectively maintained. However, the modulator is now working into a higherload impedance than before and if the variation in output is substantial the power transfer is inefllcient, since maxisented to the modulation or signal potential is maintained substantially constant.

The novel features which I believe to be char- I acteristlc of my invention are set forth in the appended claims. My invention itself, however, both as to its organization and method of operation may best be understood by reference to the following description taken in connection with the accompanying drawing which represents schematically a circuit embodying the principles of my invention.

Referring to the drawing there is illustrated a source of radio frequency excitation |li including an electron discharge device including a cathode i2, a control electrode II, and an anode l4. The output of the electron discharge device I is fed to a suitable radio frequency output circuit is which may be an additional amplifier or an antenna coupling circuit, for example. Thus, the

radio frequency electromotive force applied to the control electrode is reproduced on the anode.

There is illustrated in the drawing an anode or plate modulation system fo modulating the radio frequency current being conducted to the output stage l5. device includes a suitable radio frequency impedance indicated by the numeral l6. A source of positive operating potential is indicated by the numeral I1 and a suitable source of modulating or signal potential is indicated by the numeral ll. 7 The modulating potential may be introduced into the anode circuit by a modulation transformer l9 fed from any suitable modulation system but it will be understood that any suitable means of applying a modulating potential in series with the anode and source of anode potential may be employed.

In order to vary theradlo frequency output from the device ll one or more resistors 20 and 2| may be inserted in the anode circuit and there is provided means for varying the amount of resistance in the circuit. For this purpose I have provided a power control switch 22 having a plurality of contacts. The contact 23 is connected to the anode circuit between the element l6 and the resistor 2| Th contact 24 is connected to a point between the resistances 20 and 2| while contact 25 remains open. The movable portion 2|i oi' the switch 22' is connected to a point between the resistance 20 and the source of modulating potential l8. When the switch arm 26 engages the contact 23, the resistances 20 and 2| are shunted and no resistance is introduced into the anode circuit whereas if the switch is on contact 24 the resistance 2| is included in the anode circuit, and if the switch arm is moved to the contact 25 both resistances are included.

v'I'he sizes ,of the resistors 20 and 2| are chosen according to the percentage change in output .desired. Forexample, if the power output with the switch engaging contact 23 i considered to be output, and if the resistance 2| is chosen of such size that the anode current of the dischargedevice produce s a voltage drop therethrough equal to (l-l/\ 2) times the anode supply, then with the switch in engagement with contact 24 and the resistance 2| in the anode circuit, the output is cut down to 50%; and ifboth resistances 20 and 2| are included. in the circuit and the value of'resistance 20 is such that when added to thevalue of resistance 2| a voltage drop is produced equal to one-half the anode supply, the output is cut down to 25% of the original amount.

The anode circuit for the discharge As previously explained, with one or both resistances 20, 2! in the circuit the modulator is working into a higher load impedance than before, and assuming that the circuit had originally been properly designed so that the, impedance of the source of modulation potential I! and the anode circuit of the discharge device Ii were substantially equal, the modulator is no longer working at its best efficiency or minimum distortion. In order to avoid this undesirable condition there are provided resistors 21 and 28 which may be inserted as shunt loads on the source of modulating potential I! in such a manner as us keep the total modulator load impedance constant under all adjustments of the power control switch 22. For this purpose the switch 22 is provided with contacts 29, 3'0 and Ii so disposed that the arm 28 of the switch engages contacts 23 and 28, contacts 24 and 30, and contacts "and BI, respectively, at the same time. The contact 20 is open, the contact ill is connected to one end of the resistance 21 and the contact 3| is connected'to the other end of the resistance 21. The resistance 28 is connected between the low potential and of the modulating source I8 and the aforementioned other end of the resistance 21. Therefore, when the switch 22 is in the first position, i. e., when it engages the contacts 2! and 29, there is no resistance introduced into the anode circuit and there is no shunting resistance connected around the source l8. In the position of the switch in which the arm 26 engages the contacts 24 and 30 the resistance 2i is connected in the anode circuit and the seriesconnected resistances 21 and 28 are connected in shunt across the secondary of the transformer II. In the third position of the switch, that is, when the contacts 25 and Si areengaged by the switch 26, both resistances 20 and 2| are connected in the anode circuit but only the resistance 2! is in shunt to the secondary of the transformer l9. It will thusbe seen that by properly choosing the valued the resistances, as the series resistance increases the shunt resistance decreases whereby the total modulator load impedance is maintained substantially constant under any adjustment of the power control switch.

While I have shown and described a single embodiment of my invention it will, of course, be understood that I do not wish to be limited thereto, since modifications may be made in the circuit arrangement and in the instrumentalities employed without scope of my invention as set forth in the appended claims.

What I claim as'new and. desire to secure by Letters Patent of the United States is:

1. In combination. an electron discharge device having an anode, a cathode, and a control electrode, means for applying a radio frequency electromotive force to said control electrode for reproduction on said anode, a circuit between said anode and said cathode including a source of operating potential and a source of signal potential, means for-varying the impedance of said anode circuit to vary the radio frequency output of said device, and means for varying the impedance of said anode circuit to maintain constant the total load impedance presented to said source of signal potential.

2. In combination, an electron discharge device having an anode, a cathode, and a control electrode rneans for applying a radio frequency electromotive force to said control electrode for reproduction on said anode, a circuit between said anode and said cathode including a source of operating potential and a source of signal potential, and means for varying the radio frequency output of said device, said means comprising means for varying the impedance of said circuit to currents from both of said sources while maintaining the load on said signal source constant.

3. In combination, an electron discharge device having an anode, a cathode, and a control electrode, means for applying a radio frequency electromotive force to said control electrode for reproduction on saidanode, a circuit between said anode and said cathode including a source departing from the spirit and of operating potential and a source of signal po-, tential, andmeans for varying the voltage. applied to said anode in order to vary the radio frequency output of said tube, said last means operating undesirably to change the load on said signal source, and means for compensatingfor said changes in the load on said signal source, thereby to maintain constant said load.

4.-In combination, an electron discharge device having an anode, a cathode, and a control electrode, means for applying a radio frequency electromotive force to said control electrode for reproduction on said anode, a circuit between said anode and said cathode including'a source of operating potential and a source ofsignal potential, means for varying the amount of said resistance to vary the radio frequency output power of said device, and means for adjusting the load on said source of signal potential in a manner to present a constant total load impedance to said source of signal potential as said resistance is varied.

5. In combination, an electron discharge device having an anode, a cathode, and a control electrode; means for applying a radio frequency electromotive force to said control electrode for reproductionon said anode, a circuit between said anode and said cathode including a source of operating potential and a source of signal potential, a plurality of resistors arranged to be connected in series in said anode circuit, a plurality of resistors arranged to be connected in shunt with said source of signal potential, and means for selectively and simultaneously changing the connections of both said series 'and said shunt resistors to vary the radio frequency output of said device while maintaining substantially constant the load on said source of signal potential.

HOWARD M. CROSBY. 

